Does adding a carbon layer reduce particulate effectiveness?

I recently published tests showing that carbon actually removes VOCs. But adding that carbon layer raises a question: adding carbon means the fan has to power through yet another layer of resistance. Does that make purifier less effective at removing particulates?

Test Method

I measured how much particulate it removed with a Dylos particle counter and compared the particle counts (1) before I turned on the purifier at night and (2) the average of the last four hours before I woke up in the morning.

Carbon vs. HEPA Results

With the additional charcoal layer, the Cannon particulate effectiveness dropped 1-2%. Thus, charcoal has a very minor negative effect on the Cannon.

However, the Original DIY had a harder time powering through the extra layer. Its 0.5 micron effectiveness dropped 19%, and 2.5 micron effectiveness dropped 15%.

Conclusion: Carbon Tips

For people who need carbon (and that may not be everyone), I would recommend adding the carbon to the Cannon, but I would think twice about adding carbon to the Original. One workaround is to have one Original fan running with a HEPA and another running with a carbon filter.

As always, I’m posting the raw data and more details on the test for fellow nerds below.

Data: Cannon

Over the 10 testing days, outdoor air pollution went down significantly on 6 days–poor luck! To test whether that affected the overall estimate, I isolated the three testing days with changes in outdoor PM 2.5 concentration of less than 10 micrograms.

For those three days, the 0.5 micron results were still 95%. The 2.5 micron results were 93%, which is 2% lower than in the overall tests. Thus, the overall estimate seems to be little affected by outdoor fluctuations when averaged over all of the tests.

Data: Original DIY

Below is the data for 10 tests with the Original DIY.

One day (12/21) was an outlier because the outdoor AQI almost tripled during the test. Thus, I calculated the average with and without that outlier removed. With the outlier included, .5 microns effectiveness was 5% lower and 2.5 micron effectiveness was 2% lower.

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Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

About Smart Air

Smart Air is a social enterprise based in China, India, Mongolia and the Philippines that creates simple, no-nonsense air purifiers and provides free education to combat the impacts of air pollution.
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